The rectangular hyperbola is shown to be a poor description of the response of photosynthesis to irradiance for leaves of a winter wheat crop. A model is derived which combines a simplified description of the biochemical reactions occurring within the chloroplast with the physical diffusion of CO2 from the atmosphere. The model is a non-rectangular hyperbola and uses four parameters: Pnmax (maximum rate of net photosynthesis), Rd (rate of dark respiration), α (photochemical efficiency of photosynthesis at low light), and θ (ratio of physical to total resistance to diffusion of CO2). The model is shown to be a significant improvement on the rectangular hyperbola. It provides good fits both to our own measurements and to some data from the literature, and gives consistent and reliable estimates of the four parameters.